High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990)) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from • 9 to 19 R•r in the Earth's magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on 2 years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements which we call bursty bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 •9 ergs/R•r 2. 17 R•r and were argued to have an (inferred) scale size of 15 R•r in the Yas• direction and fast shock properties. 1DeparUnent Statistical studies have also been used to characterize plasma sheet flows. Prior to 1986 only a few of these studies addressed plasma sheet flows irrespective of substorm phase [Caan et al., 1979; Hayakawa et al., 1982; Slavin et al. 1985, 1987]. For example, Hayakawa et al. [1982] using IMP 6 data from distances of-15 R•r < XOSM < -33 R•r showed that high-speed flows (V,, > 300 km/s) can occur in the plasma sheet within 1 Re from the expected neutral sheet position defined by the Russell and Brody [1967] model, so one might infer that these flows occurred in the CPS. However, the above statistical studies either did not distinguish between the CPS and its boundary or they concentrated in the distant-tail regions [Slavin et al., 1985, 1987]. The first statistical assessment of the significance of the near-Earth CPS for magnetotail transport was made by Huang and Frank [1986]. They constructed a data base using 128-512 s resolution plasma data from the ISEE 1 satellite. They applied the criterion that high-speed flows (Vi > 150 km/s) occurring 1.5 R•r or more away from the GSM equatorial plane are in the PSBL. They found that the average speed in the CPS was low (around 50 km/s) regardless of geomagnetic activity (based on the AE index). They showed (see also Figure 2 of Huang and Frank [ 1987]) that even if high-speed flows existed in their data set, these were not representative of the average properties of the CPS. They therefore argued that even if high-speed flows of short time or spatial scales may occur in the CPS, they are statistically insignificant compared to the vast majority of the (low flow velocity) data. The above study did not attempt to assess the relative contributions of the CPS and the plasma sheet boundary to magnetotail transport. A new statistical selection criterion to distinguish between the central plasma sheet and its boundary was proposed by Baumjohann et al. [1988] (subsequently referred to as BJeta188): ...

Using 8 months of tail data obtained with the AMPTE/IRM satellite, more than 270,000 ion moments and magnetic field measurements were analyzed with respect to the occurrence rates and typical characteristics of high-speed ion flows with velocities in excess of 400 km/s. The occurrence rates in the plasma sheet boundary layer, the outer central plasma sheet and the neutral sheet neighborhood have a 4:1:2 ratio for flows of 400-600 km/s. For flows in excess of 800 km/s, there is only a minimal chance to detect them in the outer central plasma sheet but equal chances in the two other regions. For high AE the chances to detect high-speed flows in the inner central plasma sheet are greater than to find them in the plasma sheet boundary layer. In the outer central plasma sheet the high-speed flow occurrence rate is small and independent of AE. In all three regions the largest occurrence rates are found near the midnight meridian at the largest radial distances accessible to IRM. High-speed flow occurrence rates and ion densities are anticorrelated. The high-speed flows are bursty with the majority of the flows lasting less than 10 s. The occurrence of the high-speed flows is strongly peaked in the sunward direction. Virtually no tailward high-speed ion flow could be detected. About 60-70% of all high-speed flows near the neutral sheet have a dominant component perpendicular to the magnetic field and are associated with comparatively large northward and duskward magnetic field directions. At times, also appreciable duskward flow components appear. Overall, our results indicate that both the plasma sheet boundary layer and the inner central plasma sheet are important regions for the dynamics of the Earth's plasma sheet. 1.

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Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most I-t•As have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

We have studied 38 low‐latitude, dayside (0800‐1600 LT) magnetopause crossings by the AMPTE/IRM satellite to investigate the variations of key plasma parameters and the magnetic field in the magnetosheath region adjacent to the dayside magnetopause. We find that the structures of the key plasma parameters and the magnetic field and the dynamics of plasma flows in this region depend strongly on the magnetic shear across the magnetopause, that is, on the angle between the magnetosheath magnetic field and the geomagnetic field. When the magnetic shear is low (<30°), a magnetosheath transition layer, also called the “plasma depletion layer,” of 10‐min average width exists where the magnetosheath magnetic field piles up against the magnetopause. In this region the plasma density and plasma β as well as the proton and electron temperatures are lower than in the magnetosheath proper. The condition for the onset of the mirror instability is generally not met in the magnetosheath transition layer, where the plasma β often falls below 1, while it is marginally satisfied in the magnetosheath proper, where usually β>1. When the magnetic shear across the magnetopause is high (>60°), the near‐magnetopause magnetosheath is more disturbed. The magnetic field in this case does not pile up in the immediate vicinity of the magnetopause, and no systematic variations in the plasma parameters are observed in this region until the encounter of the magnetopause current layer; that is, there is no magnetosheath transition layer. Also in contrast to the low‐shear case, the mirror instability threshold is marginally satisfied throughout the magnetosheath. The plasma flow pattern in the magnetosheath region adjacent to the dayside magnetopause is also found to depend strongly on the magnetic shear across the magnetopause: the magnetosheath flow component tangential to the magnetopause is enhanced and rotates to become more perpendicular to the local magnetic field as the low‐shear magnetopause is approached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnetopause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear across the magnetopause is high. In essence, our observations provide evidence for high (low) rate of transfer of magnetic flux and mass across the magnetopause when the magnetic shear is high (low). The relationships between the electron and proton temperature anisotropies and β in the near‐magnetopause magnetosheath region are also examined. It is found that Te⊥/Te∥ remains close to 1 for the entire range of βe, whereas Tp⊥/Tp∥ is generally anticorrelated with βp∥. However, no universal relationship seems to exist between Tp⊥/Tp∥ and βp∥.